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Keeping Unvented Tiny House 25 degree gable roof dry/mold free. How to?

leavytinyroof | Posted in General Questions on

Hello there,

I’m over in Eastern Washington getting close to completion on my 18’ mini house. As me and my contractors were putting on our OSB roof deck I still hadn’t decided whether to go with a single layer or 2 layers with 1 inch of R10 foamular between. The extra weight, $1,200 cost, extra 3 inches of height over 13’ 6in, and the cosmetic fact we would have had to add another piece of LP smart trim vs the current 7.25 belly band that was flush with the single layer OSB, I decided to stick with the single layer:

*Now we’ll be putting Titanium PSU 30 ice and water shield over the OSB and then standing seam metal.

*Inside is 2×6 framing and I’m thinking Sika Rmax foam polyiso friction fit with a little sika insulation sealant at the seams. With 5.5in this would be R33. 2’’ pieces, more 2’’ pieces, and then 1.5’’ pieces. Then taped drywall to block out inside air. Then some sort of wood t & g.

The Sika Rmax has a foil side and a guy at the local building supply store told me to leave a gap between the foil and the drywall to reflect? Yes? No? If so then maybe I’d do 5in total of Rmax polyiso.

Also, a few people have recommended a small 2in aluminum vent/Louver set in every 2×6 cavity of the interior ceiling. Good idea? That seems to be counterproductive to the idea of not letting interior air up into the ceiling cavity. If air and water got through the I & W, how would it get through the Rmax and sika sealant at seams to even reach the little vents set in the drywall/t & g? I have no idea what to do and just want the best possible outcome with the standing seam metal, ice and water shield, OSB, unvented, etc.

Take care and thank you for feedback.

Cheers.

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Replies

  1. Expert Member
    KYLE WINSTON BENTLEY | | #1

    /*
    Inside is 2×6 framing and I’m thinking Sika Rmax foam polyiso friction fit with a little sika insulation sealant at the seams. With 5.5in this would be R33. 2’’ pieces, more 2’’ pieces, and then 1.5’’ pieces. Then taped drywall to block out inside air. Then some sort of wood t & g.
    */

    This will be 10x more work, cost more, and perform worse than many alternatives, even though the material itself has a higher r value.

    1. It's going to be practically impossible to cut them, install them, and seal them such that there aren't gaps between the boards, or at the seams, or that they're physically touching eachother through all space. An R23 batt installed well, of any material, will provide better insulation at 1/4 the price.

    2. There doesn't need to be an air gap between the insulation and your wall material. You want them to fit snugly together to eliminate any voids for air movement. The only time you would want an air gap between insulation and another surface would be on the roof, for a ventilation channel nearest the roof decking, with the insulation below it.

    1. leavytinyroof | | #2

      I really appreciate this response Kyle. I was just preparing to post a different question since I know the cut and cobble doesn’t have a great reputation. The next question would be regarding having 2 vapor barriers on my unvented 25 degree roof. One being the titanium psu 30 ice and water shield, and the other being the Rmax polyiso foil side on the inside. Acceptable?

      I like roxul/rockwool; it’s what’s in my walls. What is the consensus on a hybrid setup of filling the framing cavities with r23 rockwool and then solid 2” sheets of Rmax polyiso over the rafters? I have plenty ceiling height so the loss of 2” is no big deal. Shall I also leave a vent channel between the roxul and sheathing?

      Is taping the exterior OSB seams a good idea before putting down the ice and water shield?

      Take care.

    2. leavytinyroof | | #3

      Only doing r23 rockwool is a no go right because it wouldn’t stop the interior warm air from reaching the sheathing? Cheers.

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